Make port script handle empty sent_transactions table

scripts/synapse_port_db

@@ -412,6 +412,7 @@ class Porter(object):
         self._convert_rows("sent_transactions", headers, rows)
 
         inserted_rows = len(rows)
+        if inserted_rows:
             max_inserted_rowid = max(r[0] for r in rows)
 
             def insert(txn):
@@ -420,6 +421,8 @@ class Porter(object):
                 )
 
             yield self.postgres_store.execute(insert)
+        else:
+            max_inserted_rowid = 0
 
         def get_start_id(txn):
             txn.execute(